1. Field of the Invention
The present invention relates to the structure of a photosensor used for a solid state imaging device or the like.
2. Description of the Related Art
The structure of one example of a conventional photosensor is described in the following. This example is an interline CCD (Charge-Coupled Device) solid state imaging device having effective sensitivity in the visible light range. FIG. 3A is a top plan view and FIG. 3B is a cross-sectional view illustrating the pixel structure of this conventional solid state imaging device.
The interline CCD solid state imaging device has an array of pixels arranged in a grid pattern and each pixel is made up of a photodiode 4 serving as a light receiving portion and a charge transfer path 5 serving as a signal readout portion. On the surface of the photodiode 4 and charge transfer path 5 is formed a first insulating layer 6. A gate electrode 7 is formed above the charge transfer path 5. The gate electrode 7 is covered by a second insulating layer 8.
To prevent image deterioration known as “smear” caused by photoelectrons that are generated by light that is directly incident on the charge transfer path 5, a light shielding layer 1 is formed above the charge transfer path 5 via the first and second insulating layers 6 and 8.
The light shielding layer 1 has an aperture 2 formed by etching or the like, above the photodiode 4, to allow light to enter the photodiode 4. The region above the photodiode 4 surrounded by the side walls of the aperture 2 in the light shielding layer 1 is filled with a fourth insulating layer 10 of a material having a refractive index of less than 1.50 such as silicon dioxide (see, for example, JP 2000-252451A).
With about 3 μm wide pixels in a solid state imaging device of recent years, the aperture ratio of the light receiving portion is as low as 20% or less, and the usage efficiency of incident light is accordingly low. To reduce the pixel size further, the aperture width W of the light shielding layer 1 would have to be 0.5 μm or less. The problem in that case would be that the long wavelength components of visible light are cut off by the light shielding layer 1 because the aperture width is smaller than the longest wavelength of the visible light range, which is about 800 nm.
The same applies to an amplified MOS solid state imaging device that has neither CCD charge transfer area nor a light shielding layer, because, as the signal line pitch on the pixel is reduced to as small as an effective wavelength, the signal lines function as a phase grating, causing a similar problem.